So-called “stepper” motors are widely used in applications wherein the position of a mechanical element must be readily and accurately adjusted over a predetermined range of possible positions. For example, in controlling a throttle valve, the position of a valve head with respect to a valve seat must be adjustable over a range of head positions, typically from fully open to fully closed. Stepper motors are well-suited to providing such valve control. They operate by switching the polarity of the voltage on the input motor windings. Without a feedback device, it is assumed that the motor armature moves incrementally a predetermined amount (a “step”) for each change in voltage polarity.
A stepper motor can respond to a range of loads by providing a range of torque outputs at a fixed motor voltage, for example, 12 volts, for which the windings are sized. Thus, for each specific application of a stepper motor as an electromechanical actuator, the size of stepper motor selected is determined in large by the nominal operating load range to be encountered.
In some applications, however, the nominal range of operating loads may occasionally be exceeded. For example, in controlling the head position of an inlet air valve for a solid-oxide fuel cell assembly, high torque may be required when manifold pressures are high or when the valve head is nearly seated. Under such high load conditions, a stepper motor sized for the typical range of loads will overheat and lose efficiency.
A prior art approach to this problem is to provide a larger, greater-range stepper motor capable of handling both the normal range of loads and the occasional and brief extremes. However, increasing the size and range of the stepper motor increases the size and cost of the system, and may reduce control sensitivity by increasing the size of each step.
What is needed is a means for temporarily extending the torque capabilities of a stepper motor beyond its rated torque output range.
It is a principal object of the present invention to briefly and temporarily extend the torque output range of a stepper motor.
It is a further object of the invention to minimize the size of a stepper motor required for a particular range of loads.